What it is

A dual-fuel (also called hybrid heat) system pairs an electric heat pump with a gas (or propane/oil) furnace in the same system. The heat pump handles heating in milder weather where it's cheap and efficient; when it gets cold enough that the furnace becomes the better choice, the system hands off to the furnace. You get the heat pump's efficiency in shoulder seasons and the furnace's brute-force heat when it's truly cold — without electric resistance strips.

The key concept: in a dual-fuel system the furnace replaces the electric heat strips as the backup. And critically, the heat pump and the furnace do not run at the same time — it's one or the other, with a clean changeover.

How it works

Above the changeover temperature: the heat pump does all the heating. The furnace stays off.

Below the changeover temperature: the system locks out the heat pump compressor and runs the gas furnace for heat instead. The furnace's own blower and controls take over the heating call.

Why they can't run together: blowing furnace-heated air (120°F+) over the indoor coil while the heat pump is trying to use that coil as a condenser drives head pressure and discharge temps way up — hard on the compressor and pointless. So the changeover is a true switch, not a blend. The control logic ensures the W (furnace heat) call and the heat-pump compressor call are mutually exclusive below the switchover point.

What controls the changeover?

  • Outdoor thermostat / outdoor sensor: the classic method. An outdoor temperature sensor tells the control when it's crossed the set switchover temp. Above it → heat pump; below it → lock out the heat pump and enable the furnace.
  • Smart/communicating thermostats: many do dual-fuel changeover in software based on outdoor temperature (and sometimes utility rates), no separate outdoor stat needed.
  • Fossil-fuel kit: on some setups a "fossil fuel kit" relay board manages the lockout/changeover between the heat pump and the furnace, ensuring the furnace handles backup and defrost-tempering instead of strips.

During defrost in a dual-fuel system, the furnace (not electric strips) provides the tempering heat so the supply air doesn't go cold while the outdoor coil defrosts.

In the field

Set the changeover (switchover) temperature based on the economic balance point — the outdoor temperature where running the gas furnace becomes cheaper than running the heat pump. That depends on local gas price vs electric price and the equipment's efficiency, so it's not a fixed number. In some markets gas is cheap and you switch over higher (heat pump used less); where electricity is cheap relative to gas, you let the heat pump run colder before switching.

  • Practically, many installs land the switchover somewhere in the ~30–40°F range, but it should be set from the actual economics, not a default.
  • Set it too high and you burn gas when the heat pump could have run cheaper. Set it too low and you run the heat pump into territory where it's expensive/struggling.

Verify the lockout actually works:

  • Above switchover: confirm the heat pump runs on a heat call and the furnace stays off.
  • Below switchover (or simulate by adjusting the outdoor stat/sensor): confirm the heat pump compressor is locked out and the furnace fires for the heat call.
  • Confirm the furnace provides defrost tempering (not strips) during a forced defrost.

Normal values & targets

  • Switchover temperature: commonly ~30–40°F, but properly set from the economic balance point (gas vs electric cost + efficiency) — verify locally.
  • Heat pump supply air: cooler than furnace supply (~90–105°F vs 120°F+). Normal — don't "fix" it by switching to furnace early.
  • The two heat sources are mutually exclusive — never both at once.
  • Defrost tempering is provided by the furnace in dual-fuel (replaces strip heat).

Common faults & what they mean

  • Furnace and heat pump both trying to run — failed changeover/lockout logic. This is a real problem: it stacks furnace heat onto the heat pump's condenser and spikes head pressure. Find the fossil-fuel kit/outdoor stat/thermostat config fault immediately.
  • Furnace runs all winter, heat pump never does — switchover set too high, outdoor sensor failed reading "cold," or the heat pump itself is down so the system falls back to furnace. Check the sensor and whether the heat pump actually heats above switchover.
  • Heat pump runs in brutal cold, furnace never engages — switchover set too low or outdoor sensor failed reading "warm." The heat pump struggles, frosts hard, and the customer is cold.
  • Cold air during defrost — furnace not providing tempering; check the changeover/defrost wiring so the furnace (not nothing) carries defrost.
  • High bills either way — switchover not set to the economic balance point; you're using the expensive fuel for the conditions.

Tech tips & gotchas

  • The big safety/equipment rule: heat pump and furnace never run simultaneously. If you ever find both firing, stop and fix the lockout — it's hard on the compressor.
  • Switchover is an economics decision, not a comfort decision. Set it from gas-vs-electric cost and efficiency. Don't just default to 35°F because that's what the last guy used.
  • A dual-fuel thermostat must be configured for dual fuel — wrong config and it'll treat the furnace like ordinary aux (running it with the heat pump) or never switch over. Confirm the stat type and settings.
  • Wiring differs from a strip-heat heat pump. The W call goes to the furnace, and the changeover logic has to lock the compressor out. Don't wire it like a standard heat pump + air handler.
  • Remember the heat pump's cooler supply air is normal; resist the urge (or the customer's) to switch to furnace just because it "feels warmer." That defeats the efficiency the system was bought for.

Safety / code notes

  • The furnace side still follows all gas-furnace requirements: combustion air (fuel-gas/mechanical code combustion-air sections), venting by appliance category (fuel-gas code venting sections), and proper manifold pressure/CO verification with a combustion analyzer.
  • The lockout that prevents simultaneous operation is functional protection for the compressor — verify it works; don't bypass it.
  • Refrigerant-side service on the heat pump follows EPA Section 608 (recover, don't vent) and proper evacuation/charging practice.